Experimental aspects of ¹⁴N overtone RESPDOR solid-state NMR spectroscopy under MAS beyond 60 kHz

Nitrogen-14 (¹⁴N) overtone (OT) spectroscopy under fast magic angle spinning (MAS) conditions (>60 kHz) has emerged as a powerful technique for observing correlations and distances between ¹⁴N and ¹H, owing to the absence of the first-order quadrupolar broadenings. In addition, ¹⁴N^OT allows selective manipulation of ¹⁴N nuclei for each site. Despite extensive theoretical and experimental studies, the spin dynamics of ¹⁴N^OT remains under debate. In this study, we conducted experimental investigations to assess the spin dynamics of ¹⁴N^OT using the rotational-echo saturation-pulse double-resonance (RESPDOR) sequence, which monitors population transfer induced by a¹⁴N^OT pulse. The ¹⁴N^OT spin dynamics is well represented by a model of a two-energy-level system. Unlike spin-1/2, the maximum excitation efficiency of ¹⁴N^OT coherences of powdered solids, denoted by p, depends on the radiofrequency field (rf-field) strength due to orientation dependence of effective nutation fields even when pulse lengths are optimized. It is also found that the p factor, contributing to the ¹⁴N^OT spin dynamics, is nearly independent of the B ₀ field. Consequently, the filtering efficiency of RESPDOR experiments exhibits negligible dependence on B ₀ when the ¹⁴N^OT pulse length is optimized. The study also identifies the optimal experimental conditions for ¹⁴N^OT/¹H RESPDOR correlation experiments.